Hide / Show Custom field depending of the page ID (Magic Field 2)

You can remove meta boxes with the remove_meta_box function:

function wpse50430_remove_meta_boxes() {

    // make sure we're on an admin screen and `post` is set
    if( !is_admin() && !isset( $_GET['post'] ) )
        return;

    if( $_GET['post'] == 99 ): // editing page ID 99
        remove_meta_box( 'pageparentdiv', 'page', 'normal' );
    endif;

}
add_action( 'admin_menu', 'wpse50430_remove_meta_boxes' );

To remove meta boxes added by plugins, you have to find the handle ID. If you use Chrome’s developer tools, you can right-click the meta box and look for the ID of the element with class postbox.